Modern IP (Internet Protocol) networks carry traffic data of numerous time-sensitive applications, such as video conferencing and interactive multiplayer games. Consequently, a network is required to offer a very high level of service quality (QoS) and availability. It is no surprise, then, that service providers go to great lengths to guarantee that their networks perform satisfactorily in those respects. Nevertheless, unexpected component failures as well as scheduled maintenance procedures often result in periods of degraded network performance.
One of these periods of degraded network performance are called convergence periods, because they correspond to the time needed by the network's Interior Gateway Protocol (IGP), e.g. Open Shortest Path First (OSPF), to converge after a topology change takes place. More specifically, within each such period every router updates its forwarding information base (FIB) tables independently, as information about the topology change is propagated through the network. Consequently, forwarding tables of different routers may be temporarily inconsistent with each other, leading to traffic being caught in forwarding loops and/or discarded.
Convergence periods typically last for hundreds of milliseconds, or even seconds. Unfortunately, such a long recovery time is not acceptable for most real-time applications. By contrast, a sub-50-millisecond recovery time, like that of SONET/SDH and MPLS networks, would be desirable to ensure that consumers hardly notice any adverse effects.
On a related note, energy efficiency has recently become an important concern for service providers. One popular technique to reduce energy consumption involves dynamically dimensioning the network to meet the current traffic demand, for example by temporarily switching off unneeded links and routers. Like any other topology change, however, these operations are liable to have a negative impact on network performance, and may occur rather frequently. Therefore, attaining a low recovery time from such events gains even greater significance, because without it the aforementioned energy-saving measures are unlikely to be widely adopted.